IS31LT3352 40V LED DRIVER WITH TEMPERATURE COMPENSATION OCTOBER 2011 GENERAL DESCRIPTION The IS31LT3352 is a continuous mode inductive step-down converter, designed for driving a single LED or multiple series connected LEDs efficiently from a voltage source higher than the required LED voltage. The chip operates from an input supply between 6V and 40V and provides an externally adjustable output current of up to 750mA. Depending upon supply voltage and external components, this can provide up to 30 watts of output power. The IS31LT3352 includes an integrated output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. The IS31LT3352 integrates temperature compensation function in order to maintain LEDs’ stable and reliable operation. The IS31LT3352 measures the thermistance mounted close to LEDs. When ambient temperature near LEDs goes too high and the Negative Temperature Coefficient thermistors reach the value of threshold resistance connected at RTH pin, output current starts to reduce automatically. After the ambient temperature falls down to a safe temperature,the current will return to the set value. The IS31LT3352 can be connected as LED drivers’ chain with the same temperature compensation percentage. In this chain, every IS31LT3352’s ADJO output pin drives next stage’s IS31LT3352 ADJI input pin with temperature compensation information. So, only one thermistor is needed in the whole IS31LT3352 system. FEATURES Simple low parts count Internal 40V power switch Wide input voltage range: 6V to 40V Up to 750mA output current High efficiency (up to 95% ) 1200:1 dimming ratio Typical 5% output current accuracy Single pin on/off and brightness control using DC voltage or PWM Up to 1MHz switching frequency Inherent open-circuit LED protection Thermal shutdown to protect IC itself Temperature compensation to protect LEDs APPLICATIONS Low voltage halogen replacement LEDs Automotive lighting Low voltage industrial lighting LED back-up lighting Illuminated signs APPLICATION CIRCUIT IS31LT3352 Copyright © 2011 Integrated Silicon Solution, Inc. All rights reserved. ISSI reserves the right to make changes to this specification and its products at any time without notice. ISSI assumes no liability arising out of the application or use of any information, products or services described herein. Customers are advised to obtain the latest version of this device specification before relying on any published information and before placing orders for products. Integrated Silicon Solution, Inc. does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless Integrated Silicon Solution, Inc. receives written assurance to its satisfaction, that: a.) the risk of injury or damage has been minimized; b.) the user assume all such risks; and c.) potential liability of Integrated Silicon Solution, Inc is adequately protected under the circumstances Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 1 IS31LT3352 SYSTEM APPLICATION IS31LT3352 IS31LT3352 IS31LT3352 Note: Each IS31LT3352 can driver up to three slave chips in the next stages, and it is recommend no more than three stages are used for the current coherence, therefore, up to thirteen IS31LT3352s are allowed in one system Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 2 IS31LT3352 PIN CONFIGURATIONS Package Pin Configurations SOP8 PIN DESCRIPTION Pin NO. Name VIN 1 ISENSE 2 RTH 3 RNTC 4 ADJO 5 ADJI 6 GND LX 7 8 Description Input voltage (6V to 40V). Decouple to ground with 1μF or higher X7R ceramic capacitor close to device Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom =0.1/RS The pin set the starting temperature of temperature compensation by connecting an external resistor. The output currents reduction slope set pin by connecting an external thermistor in temperature compensation mode. LED drivers chain application pin. * When R3(NTC)>R2,VADJO=VADJI * When R3(NTC)<R2, ADJO pin outputs ADJI voltage with temperature compensation information VADJIO=VADJI*R3/R2 Multi-function On/Off and brightness control pin: * Leave floating for normal operation.(VADJI = VREF = 1.2V giving nominal average output current IOUT nom=0.1/RS ) * Drive to voltage below 0.2V to turn off output current * Drive with DC voltage (0.3V<VADJI <1.2V) to adjust output current from 25% to 100% of IOUTnom * Drive with PWM signal to adjust output current. *When driving the ADJI pin above 1.2V, the current will be clamped to 100% brightness automatically. Ground (0V) Drain of power switch Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 3 IS31LT3352 ORDERING INFORMATION Industrial Range: -40°C to +85°C Order Number Quantity per reel IS31LT3352-V1GRLS2-TR 2,500 IS31LT3352-V2GRLS2-TR 2,500 Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 Package SOP-8, Lead-free SOP-8, Lead-free VSENSE 91mV to 101mV 99mV to 110mV 4 IS31LT3352 ABSOLUTE MAXIMUM RATINGS (NOTE 1) Symbol Parameter Rating VIN Input voltage -0.3V to +50V VISENSE ISENSE voltage VLX LX output voltage VADJ ,VADJO, Rth, RNTC VIN+0.3V to VIN-5V ,VIN>5V VIN+0.3V to -0.3V,VIN<5V -0.3V to +50V Pin input voltage -0.3V to +6V ILX Switch output current 800mA Ptot Power dissipation 1.2W TOP Operating temperature -40 to 85°C TST Storage temperature -55 to 150°C Tj MAX Junction temperature 150°C RθJA Junction to ambient 80°C/W ESD Susceptibility(human body mode) 2kV ELECTRICAL CHARACTERISTICS (VIN=12V, Tamb=25°C unless otherwise stated) (NOTE 2) Symbol VIN IINQoff IINQon VISENSE VSENSEHYS Parameter Conditions Input voltage Quiescent supply current with output off Quiescent supply current with output switching Mean current sense threshold voltage ADJI pin grounded 40 ADJI pin floating Measured on ISENSE pin with respect to VIN ADJI pin floating 60 80 μA 450 600 μA IS31LT3352-V2 99 105 110 ±15 Internal reference voltage Measured on ADJI pin with pin floating VOS V 101 VREF DC voltage on ADJI pin to switch chip from quiescent (off) state to active (on) state RTH and RNTC pin offset voltage 40 95 VSENSE =0.1V VADJIon Unit 91 ISENSE pin input current VADJIoff Max. IS31LT3352-V1 Sense threshold hysteresis External control voltage range on ADJI pin for dc brightness control DC voltage on ADJI pin to switch chip from active (on) state to quiescent (off) state Typ. 6 ISENSE VADJI Min. 8 % 10 1.2 0.3 mV μA V 1.2 V VADJI falling 0.15 0.2 0.25 V VADJI rising 0.2 0.25 0.3 V Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 10 mV 5 IS31LT3352 ELECTRICAL CHARACTERISTICS (VIN=12V, Tamb=25°C unless otherwise stated) (NOTE 2) (continued) Symbol ILX(leak) VADJO Parameter Conditions Min. Typ. LX switch leakage current No temperature compensation ADJI pin floating ADJO terminal voltage Max. Unit 1 μA V 1.20 IADJO=30μA RLX LX Switch ‘On’ resistance 0.9 ILXmean Continuous LX switch current 0.65 A RADJI Resistance between ADJI pin and VREF 500 KΩ DPWM(LF) DPWM(HF) fLX VADJO to VADJI offset Brightness control range at low frequency PWM signal PWM frequency =100Hz PWM amplitude=5V,Vin=15V, L=27uH, Driving 1 LED 1200:1 Brightness control range at high frequency PWM signal PWM frequency =10kHz PWM amplitude=5V,Vin=15V, L=27uH, Driving 1 LED 13:1 Operating frequency ADJI pin floating L=100μH (0.82Ω) IOUT=350mA @ VLED=3.4V Driving 1 LED 154 ADJI pin to ADJO pin drift voltage -38 1.5 Ω KHz 38 mV TONmin Minimum switch ‘ON’ time LX switch ‘ON’ 200 ns TOFFmin Minimum switch ‘OFF’ time LX switch ‘OFF’ 200 ns fLXmax Recommended operating frequency maximum 1 MHz DLX Recommended duty cycle range of output switch at fLXmax TPD Internal comparator propagation delay 50 ns TSD Thermal shutdown temperature 140 °C Thermal shutdown hysteresis 20 °C TSD-HYS 0.3 0.7 0.9 NOTES: 1. Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. 2. All parts are production tested at TA=25°C. Other temperature limits are guaranteed by design. Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 6 IS31LT3352 TYPICAL OPERATING CONDITIONS For typical application circuit ,at Tamb=25°C unless otherwise stated. 1 6 1LED 4LED 5LED 0.8 6LED 7LED 0.7 8LED 9LED 0.6 10LED D e v il a t io n fr o m n o m in a l c u rr e n t( % ) Efficiency (%) 3LED 1LED 5 2LED 0.9 2LED 4 3LED 3 4LED 5LED 2 6LED 1 7LED 8LED 0 9LED -1 0.5 5 10 15 20 25 30 35 10LED -2 40 5 10 15 1LED 2LED 0.9 3LED Effiency (%) 4LED 0.8 5LED 6LED 7LED 0.7 8LED 9LED 0.6 10LED 0.5 15 20 25 30 35 40 Vin(V) Efficiency vs. No. of LEDs L=47uH, Rs=0.33Ohm Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 Devilation from nominal current(%) 1 10 25 30 35 40 Output current variation with Supply Voltage L=100uH,Rs=0.33Ohm Efficiency vs. No. of LEDs L=100uH, Rs=0.33Ohm 5 20 V in ( V) Vin(V) 7 6 1LED 5 2LED 4 3LED 4LED 3 5LED 2 6LED 1 7LED 0 8LED -1 9LED -2 10LED -3 5 10 15 20 25 30 35 40 Vin(V) Output current variation with Supply Voltage L=47uH, Rs=0.33Ohm 7 IS31LT3352 Typical operating conditions (continued) 1.4 1.201 1.2 1.2005 Vref(V) Vref(V) 1 1.2 1.1995 0.8 0.6 0.4 1.199 0.2 0 1.1985 5 10 15 20 25 30 35 0 40 2 4 Vin(V) Vref vs. Vin over nominal supply voltage range 8 10 Vref vs. Vin at low supply voltage 600 120 500 100 400 80 Iin(uA) Iin(uA) 6 Vin(V) 300 200 100 60 40 20 0 0 0 5 10 15 20 25 30 35 40 Vin(V) 0 5 10 15 20 25 30 35 40 Vin(V) Supply Current vs. Vin (Operating) Shutdown Current vs. Vin (Quiescent) 1.4 1.2 Vadjo(V) 1 0.8 0.6 0.4 0.2 0 0 300 600 900 1200 Rntc(ohm) Vadjo vs. Rntc Rntc falling, Rth=1kohm Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 8 IS31LT3352 APPLICATION INFORMATION Setting nominal average output current with external resistor RS The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (RS) connected between VIN and ISENSE and is given by: Output current adjustment by PWM control Directly driving ADJI input A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJI pin, as shown below, to adjust the output current to a value below the nominal average value set by resistor RS, the signal range from 0V~5V. The PWM signal must have the driving ability to drive an internal 500KΩ pull-up resistor. IOUT nom = 0.1/RS [for RS>0.13Ω] The table below gives values of nominal average output current for several preferred values of current setting resistor (RS) in the typical application circuit shown on page 1: RS (Ω) Nominal average output current (mA) 0.13 769 0.15 667 0.27 0.3 370 333 Vsense is divided into two ranges to improve current accuracy, please refer to bin information on page 4. The above values assume that the ADJI pin is floating and at a nominal voltage of VREF =1.2V. Note that RS=0.13Ω is the minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value. It is possible to use different values of RS if the ADJI pin is driven from an external voltage. Output current adjustment by external DC control voltage The ADJI pin can be driven by an external dc voltage (VADJI), as shown, to adjust the output current to a value below the nominal average value defined by RS. DC ADJI IS31LT3352 GND GND The nominal average output current in this case is given by: IOUTdc = 0.083*VADJI/RS [for 0.3V< VADJI <1.2V] Note that 100% brightness setting corresponds to VADJI = VREF. When driving the ADJI pin above 1.2V, the current will be clamped to 100% brightness automatically. The input impedance of the ADJI pin is 500k ±25%. Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 Driving the ADJI input from a microcontroller Another possibility is to drive the chip from the open drain output of a microcontroller. The diagram below shows one method of doing this: K10 MCU ADJI IS31LT3352 GND GND The diode and resistor suppress possible high amplitude negative spikes on the ADJI input resulting from the drain-source capacitance of the FET. Negative spikes at the input to the chip should be avoided as they may cause errors in output current or erratic device operation. Shutdown mode Taking the ADJI pin to a voltage below 0.2V will turn off the output and supply current will fall to a low standby level of 60μA nominally. Inherent open-circuit LED protection If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the chip, so the chip will not be damaged, unlike in many boost converters, where the back EMF may damage the internal switch by forcing the drain above its breakdown voltage. Capacitor selection A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. 9 IS31LT3352 If the source is a DC supply, the capacitor is decided by ripple of the source, the value is given by: I *T C min F on U MAX IF is the value of output current, U MAX is the ripple of power supply. Ton is the “ON” time of MOSFET. The value is normally 2 times of the minimum value. If the source is an AC supply, typical output voltages ripple from a nominal 12V AC transformer can be ±10%.If the input capacitor value is lower than 200μF, the AC input waveform is distorted, sometimes the lowest value will be lower than the forward voltage of LED strings. This will lower the average current of the LEDs. So it is recommended to set the value of the capacitor bigger than 200uF. For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application and should not be used. Inductor selection Recommended inductor values for the IS31LT3352 are in the range 47μH to 220μH. Higher values of inductance are recommended at higher supply voltages and low output current in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range. (See graphs). The inductor should be mounted as close to the chip as possible with low resistance connections to the LX and VIN pins. The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. It is recommended to use inductor with saturation current bigger than 1.2A for 700mA output current and inductor with saturation current bigger than 500mA for 350mA output current. The inductor value should be chosen to maintain operating duty cycle and switch 'on/off' times within the specified limits over the supply voltage and load current range. The following equations can be used as a guide. LX Switch 'Off' time TOFF V LED L I V D I AVG ( rL R S ) Note: TOFFmin>200ns Where: L is the coil inductance (H) rL is the coil resistance (Ω) Iavg is the required LED current (A) ∆I is the coil peak-peak ripple current (A) {Internally set to 0.3 × Iavg} VIN is the supply voltage (V) VLED is the total LED forward voltage (V) RLX is the switch resistance (Ω) VD is the diode forward voltage at the required load current (V) Example: For VIN=12V, L=47μH, rL=0.64Ω, VLED=3.4V, Iavg =333mA and VD =0.36V TON = (47e-6 × 0.105)/(12 - 3.4 - 0.612) = 0.62μs TOFF = (47e-6 × 0.105)/(3.4 + 0.36 + 0.322)= 1.21μs This gives an operating frequency of 546kHz and a duty cycle of 0.34. Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal supply voltage. This helps to equalize the undershoot and overshoot and improves temperature stability of the output current. Diode selection For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the maximum operating voltage and temperature. If alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. It is very important to consider the reverse leakage of the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device. The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the LX output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the LX pin including supply ripple, does not exceed the specified maximum value. LX Switch 'On' time TON V IN V LED L I I AVG ( R S rL R LX ) Note: TONmin>200ns Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 Reducing output ripple Peak to peak ripple current in the LED can be reduced, if required, by shunting a capacitor Cled across the LED(s) as shown below: 10 IS31LT3352 Rs Vin LED L1 D1 VIN CLED ISENSE LX IS31LT3352 A value of 1μF will reduce nominal ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of LED voltage. Operation at low supply voltage The internal regulator disables the drive to the switch until the supply has risen above the startup threshold set internally which makes power MOSFET on-resistance small enough. Above this threshold, the chip will start to operate. However, with the supply voltage below the specified minimum value, the switch duty cycle will be high and the chip power dissipation will be at a maximum. Care should be taken to avoid operating the chip under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (See next section on thermal considerations). Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient to prevent the chip from switching below approximately 6V. This will minimize the risk of damage to the chip. Thermal considerations When operating the chip at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. Note that the chip power dissipation will most often be a maximum at minimum supply voltage. It will also increase if the efficiency of the circuit is low. This may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output. minimize the change in output current when no compensation is employed. If output current compensation is required, it is possible to use an external temperature sensing network - normally using Negative Temperature Coefficient (NTC) thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network can reduce output current with increasing temperature through internal circuit. As shown in the figure below, the temperature compensation curve is decided by R1, NTC thermistor R2 and resistor R3. When LED(s) temperature increases, thermistance of R2 starts to reduce. As R2 reduces to the point that R2’s thermistance plus R3 resistance equaling to R1 resistance, temperature compensation function starts to work and Iout starts to reduce. The Iout current with temperature compensation’s equation is: In the case that 0.3< VADJI <1.2V: IOUTdc = 0.083*VADJI (R2+R3)/R1*RS In the case that VADJI >1.2V: IOUTdc = 0.1*(R2+R3)/R1*RS R2 and R3 decide the temperature compensation slope, if R3 is just 0ohm, slope is only decided by thermistor R2’s parameter B-constant. And larger R3’s resistance results in slope more even. If the temperature compensation threshold is selected, larger R2 and R3 selected need larger R1 to match and vice versa. Too large R1 make Rth pin more sensitive to noise, too small R1 will make IC current consumption larger. From 1K to 100K of R1 is recommended. RNTC R3 RTH R2(NTC) ADJO IS31LT3352 GND R1 An IS31LT3352 calculator is available from the ISSI to assist with temperature compensation design and here are some detail examples as below: Temperature compensation of output current High luminance LEDs often need to be supplied with a temperature compensated current in order to maintain stable and reliable operation at all drive levels. The LEDs are usually mounted remotely from the chip. For this reason, the temperature coefficients of the internal circuits for the IS31LT3352 have been optimized to Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 11 IS31LT3352 400 400 350 350 Current (mA) 300 Current (mA) 300 250 200 150 250 200 150 100 100 50 50 0 0 0 0 20 40 60 80 100 120 140 160 20 40 60 80 100 120 140 160 L E D A m b i e n t T e m p ( ℃) L E D A m b i e n t T e m p ( ℃) B=4485, R1=58.6k, R2=100k, R3=10k B=4485, R1=48.6k, R2=100k, R3=0R 400 350 Current (mA) 300 250 200 150 100 50 0 0 20 40 60 80 100 120 140 160 140 160 L E D A m b i e n t T e m p ( ℃) B=4485, R1=20.6k, R2=100k, R3=0R 400 350 Current (mA) 300 250 200 150 100 50 0 0 20 40 60 80 100 120 L E D A m b i e n t T e m p ( ℃) B=4485, R1=22.1k, R2=220k, R3=0R Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 12 IS31LT3352 LAYOUT CONSIDERATIONS LX pin The LX pin of the chip is a fast switching node, so PCB traces should be kept as short as possible. To minimize ground 'bounce', the ground pin of the chip should be soldered directly to the ground plane. Coil and decoupling capacitors It is particularly important to mount the coil and the input decoupling capacitor close to the chip to minimize parasitic resistance and inductance, which will degrade efficiency. It is also important to take account of any trace resistance in series with current sense resistor RS. High voltage traces Avoid running any high voltage traces close to the ADJI pin, to reduce the risk of leakage due to board contamination. Any such leakage may raise the ADJI pin voltage and cause excessive output current. A ground ring placed around the ADJI pin will minimize changes in output current under these conditions RTH, RNTC pin The PCB trace from R1 to RTH pin should be as short as possible to reduce noise pickup. Because NTC thermistor R2 is mounted close to the LEDs and remote from IS31LT3352, the PCB trace from R2 to RNTC pin will be longer and pick up noise more easily. A 0.1uF capacitor from RNTC pin to ground and close to the RNTC pin is recommended to filter the frequency noise and provide protection against high voltage transients. ADJO pin Because ADJO pin drives next stages, ADJI pins and the PCB trace may be longer which picks up noise easily. In this case 200pF (max) capacitor is needed to connect from ADJO trace to ground to filter out the noise. Best practice is to connect one capacitor respectively close to ADJO output pin and the next stage ADJI input pins, but the total capacitance besides the parasitic capacitance from ADJO pin to ground must be less than 200pF. Please refer to the connection as below. ADJI pin The ADJI pin is a high impedance input, so when left floating, PCB traces to this pin should be as short as possible to reduce noise pickup. The ADJI pin is a high impedance input, so when left floating, PCB traces to this pin should be as short as possible to reduce noise pickup. ADJI pin can also be connected to a voltage between 1.2V~5V. In this case, the internal circuit will clamp the output current at the value which is set by ADJI=1.2V. Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 13 IS31LT3352 PACKAGE INFORMATION SOP-8 Integrated Silicon Solution, Inc. – www.issi.com Rev. A, 09/01/2011 14